Is an array in a member initializer list for a variadic struct possible?

Question

I created a variadic struct based on a std::tuple.

Then, I would like to use a C-style array in the member initializer list with automatic type deduction / class template argument deduction.

I created a minimum reproducible example:

#include <iostream>
#include <tuple>

template <typename ... Args>
struct VariadicStruct {
    std::tuple<Args...> data{};
    VariadicStruct() {}
    VariadicStruct(Args...args) {
        data = std::make_tuple(args...);
    };
    // More data and functions
    // . . .
};
int main() {
    VariadicStruct vs1{ {1,2,3}, 4.0, 'c' }; // Does not work
    std::cout << std::get<0>(vs1.data)[0] << '\t' << std::get<1>(vs1.data) << '\t' << std::get<2>(vs1.data) << '\n';

    VariadicStruct vs2{ 1, 4.0, 'c' };   // Fine
    std::cout << std::get<0>(vs2.data) << '\t' << std::get<1>(vs2.data) << '\t' << std::get<2>(vs2.data) << '\n';
}

I was hoping to create an array in the std::tuple using a nested braced initializer list. So, in the end, a std::tuple with int[3], doubleand char types.

My guess: Automatic type deduction does not work here.

Is there any way to make this happen?

(This is not a duplicate of this)

I guess that this is the answer, why it does not compile. Just to confirm: is this not a `std::initializer_list`? Maybe not. But is there any other solution possible to achieve, what I want? — A M, Feb 01 '23 at 07:26
btw no need for `make_tuple`, use the initializer list `VariadicStruct(Args...args) : data(args...) {};` and you probably want perfect forwarding, rather than pass by value (which wouldnt work with a c-array anyhow) — 463035818_is_not_an_ai, Feb 01 '23 at 07:44
note that there is a special rule for `auto` that makes `auto a = {1,2,3};` work and deduces `std::initializer_list`, but thats also explained in the dupe — 463035818_is_not_an_ai, Feb 01 '23 at 07:53
@HolyBlackCat: `std::array{1,2,3}` looks like the best solution for my needs. Thank you very much (to all of you). — A M, Feb 01 '23 at 08:10

score 0 · Accepted Answer · answered Feb 01 '23 at 08:43

With the very good comments from the community, I now implemented the code like the below:

#include <iostream>
#include <tuple>
#include <array>
#include <utility>

template <typename ... Args>
struct VariadicStruct {

    std::tuple<Args...> data{};

    VariadicStruct() {}
    VariadicStruct(Args...args) : data(std::forward<decltype(args)>(args)...)  {};

    // More data and functions
    // . . .
};
int main() {
    VariadicStruct vs1{ std::array{1,2,3}, 4.0, 'c' };
    std::cout << std::get<0>(vs1.data)[0] << '\t' << std::get<1>(vs1.data) << '\t' << std::get<2>(vs1.data) << '\n';
}

Is an array in a member initializer list for a variadic struct possible?

1 Answers1